Lithium ion secondary batteries that are charged and discharged by the movement of lithium ions between positive and negative electrodes have become more important as an electric power source for personal computers and portable devices, and particularly as a preferred electric power source to be mounted in vehicles, because they are light weight and can provide high energy density.
Because lithium ion secondary batteries generally tend to suffer a decline in battery performance when repeatedly subjected to charging and discharging, batteries with excellent cycling characteristics that further reduce the decline in battery performance resulting from repeated charging and discharging are needed. Patent Literatures 1 to 4 can be noted as technical documents relating to lithium ion secondary batteries. Patent Literature 1, for example, discloses technology that attempts to improve cycling characteristics by prescribing characteristics relevant to separator displacement. Patent Literature 2 discloses technology that attempts to reduce the decline in battery performance and deformation of the electrode assembly caused by swelling of the separator through setting the elastic modulus of the separator at or below a given value. Patent Literature 3 discloses technology that attempts to improve cycling characteristics by prescribing the rate of change in the thickness of the separator.